On the evolution of the lunar core

Details On the evolution of the lunar core On the evolution of the lunar core

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.525r&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 525

Computer Science

Chemical Composition, Gravitational Effects, Lunar Core, Lunar Evolution, Paleomagnetism, Heat Sources, Kreep, Lunar Rocks, Magnetic Fields, Temperature Effects

Scientific paper

Strong magnetic, dynamic, and chemical evidence now exists for a lunar Fe core, in which a dipole magnetic field was generated by the dynamo process sometime between 4.0 Ga and 3.2 Ga. The first argument that was presented requiring a core was that the nonhydrostatic second harmonic term in the lunar figure is maintained by soild-state convection and is not a relic of a primeval distortion retained by finite strength. There is evidence that a one-cell convection predominated in the early history of the Moon. First, the successive polar reorientations between 4.0 Ga and 3.8 Ga discovered from palaeomagnetism would not have been possible if the Moon's gravitational field had then a second harmonic term. Second, evidence for the absence of the latter exists in the palaeoselenoid 'frozen into' the maria surfaces. Third, a one-cell convection seems to be the explanation of the thicker farside highland crust. It is therefore suggested that there was no core prior to about 4.0 Ga and this appears to be the simplest explanation of the paleointensity curve, the magnetic field strength derived from laboratory studies of the Apollo rocks. This suggestion would imply a cold origin of the Moon, with the magma ocean only 200 km deep, the heat source for which was limited to shallow depths as in the proposal of induction by an intense primeval solar wind. Radioactive heating especially by U-235 in the first 500 m.y. would have raised the temperature of the originally cold interior to the KREEP threshold, in which the initially uniformly distributed Fe and FeS would have been convected to form a core.

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